In the above-described type of apparatus (see Industrie-Anzeiger No. 20 of Mar. 8, 1984/10 yr. pages 16 and 17) tubular hollow parts of cold-deformable metal, for instance 16 MnCr 5, are deformed by the application of high internal hydrostatic pressure. Added to the high hydrostatic internal pressure there is a particular axial pressure that is effective on the end surfaces. This axial pressure and the simultaneous effect of the internal pressure have the result that the wall of the hollow body conforms to the surface of the mold or die.
In practice a straight tube is positioned in the separation plane between upper and lower die halves and the die is closed. Between the upper and lower die halves there is however sufficient space for two diametrally opposite coaxial arranged horizontal rams whose free end faces confront and engage the tube to be deformed. The deformation takes place by feeding pressurized fluid into the interior of the tube while simultaneously exerting axial pressure by pushing the two rams toward each other.
With the known hydrostatic deformation it is possible to produce parts with uniform shape around their circumference, parts with sectoral deformation, and also parts with uniform and sectoral deformation combined.
The advantage of hollow parts produced in this manner is primarily that, as for instance with chill casting, undercut internal spaces can be produced which could not be produced by machining or could only be done with complicated tools (for instance by spark erosion). In addition the known hollow parts--in contrast to hollow parts produced by machining--are relatively light and as a result of the cold forming from the deforming when the fibers are properly oriented are particularly strong like forged goods.
Nonetheless the known internal high-pressure deformation has been found disadvantageous because a certain minimum thickness of the wall of the hollow body cannot be exceeded. This is mainly due to the fact that the tubular bodies must be made stiff enough to resist a relatively high pressure exerted on their ends, which can only be achieved with a certain minimum wall thickness.
In addition the known internal high-pressure deformation can only be used with parts where the axial force coincides exactly to the centerline of the tube and of the die. In this manner it is possible to produce substantial lateral sectoral deformations for producing, for example, crosspieces or T-pieces. In this case the longitudinal axis extends in accordance with the die's sectoral produced outward deformation transversely to the common force lines of the rams and of the tube (see Industrie-Anzeiger op. cit. page 17, FIGS. 4 and 8).
With the known internal high-pressure deformation it is possible to produce a certain number of shapes which however always lie within the framework of common lines of force of the rams and the tube to be deformed, thus for a straight basic shape.
Finally the known device (see Industrie-Anzeiger op. cit.) is disadvantageous because it is difficult to operate as a result of its rigidity required by the axial forces of the rams.